I don't think that example 2.16 is correct.
It seems to me that any dataset with a consistent default graph is consistent.
Any interpretation of the default graph can be extended to an interpretation
of the dataset by mapping every resource to inconsistent graphs. This
trivially satisfies "IGEXT(/I_d /(/n/)) is defined and /E/-entails g" as an
inconsistent graph entails every graph.
I don't think that this means that 2.16 is not tricky. There is some
interaction. However, this means that named graphs are not independent from
each other. In fact, there is a much easier situation showing that named
graphs are not independent, namely 2.13 T11.2 Similarly, it means that named
graphs are not independent from the default graph.
It is also the case that an inconsistent default graph makes the named graphs
irrelevant.
This last is, I think, a particularly strong point against providing this sort
of semantics at all.
peter